Recently, radio interference has easily occurred in wireless communications due to increasing numbers of wireless communication lines. For this reason, wireless communications have been implemented using beam-forming technologies. In the beam-forming technology, the directivity is enhanced using an antenna for arranging multiple antenna elements in an array so as to transmit high radio waves towards a specific direction alone, thus suppressing radio interference. In general, the beam-forming technology for carrying out wireless communication in a specific direction is designed such that the interval of distance between a reflector and each antenna element is set to about one quarter of a wavelength, and therefore it is possible to intensify radio waves in a desired direction by way of the reflector configured to reflect part of radio waves emitted by antenna elements.
Patent Literature 1 discloses a technology for reducing a surface current on a ground plane mesh for an antenna. This technology uses a reflector which serves as a high-impedance surface controlled in surface impedance by way of periodical structures, so as to control phases of reflective waves at the reflector, thus reducing the distance between the reflector and each antenna element to be smaller than one quarter of a wavelength. Patent Literature 2 discloses a technology for realizing an antenna whose height can be lowered due to a wavelength reducing effect by use of a magnetic substance or a dielectric substance interposed between a dipole antenna and a reflector. Patent Literature 3 discloses an antenna device including a dielectric substrate having parallel surfaces for arranging radiating elements and a ground plane. In the antenna device, the dielectric substrate indicates anisotropy of a dielectric constant in a direction perpendicular to the extended direction of each radiating element having a linear shape. In addition, the dielectric substrate has multiple metal inclusions (or split rings) aligned perpendicular to the ground plane.